THIS IS AN IN DEPTH GUIDE TO THE TRAXXAS E-REVO, BRUSHLESS EDITION E-REVO & NITRO REVO CONVERSIONS, WITH ANSWERS TO MANY OF THE FREQUENTLY ASKED QUESTIONS THAT NEWER OWNERS AND LESS EXPERIENCED HOBBYISTS MAY HAVE.

E-Revo & BL E-Revo
Stock E-revo uses a monocoque type molded chassis with integrated battery compartments, greatly increasing the overall strength and rigidity of the truck. The downside is that you are rather limited on the type and size of batteries that you can use. 7 cell side by side nimh packs fit fine, but you are restricted to 8000mah 2s, or 5000mah 3s lipo packs. In theory this sounds fine, but in reality if you are using a powerful BL motor and esc setup, you are forced into buying pricey high spec packs made from Enerland cells; you may not get such good results from over spec�d cheap Chinese cells that perform well with stock electronics or lower powered BL systems. Although the chassis is made from plastic so far there it has proven itself to be a quite robust design, thanks largely to the clever and rather beautiful skid plate that runs the full length of the chassis� underside, tying it together with the bulks and skids. One potential weak area that has become apparent is the rear portion of the chassis that links to the bulkheads is prone to excessive flex if you jump the truck or bash hard. There is a fairly simple fix for this though & requires minimal modification; a spare suspension tie-rod can be bolted to the base of the wing mount & then bolted to a small plate attached over the rear shock-tower, thus stiffening up the chassis greatly. Up-top, the esc and radio are mounted on the battery compartments, a clever idea since there is plenty of room this way, but locating the esc in the correct place is critical as the battery leads may not reach if they are a little short. The BL Erevo has its MMM esc mounted in the best place to ensure the battery wires will reach- there are dual input connectors for running a pair of lipos in series ( no extra harness required ordinarily ). It is also worth noting that the holes in the battery compartments for the battery wires to exit may not line up perfectly with the battery leads, where they exit the lipo/ nimh packs, so a little light modification may be required.

Conversion
There are two options, the 2.5 and 3.3 spec revo chassis. The 3.3 would be favourable since the longer wheelbase makes for better handling, and more room to mount the electronics etc. As for the specific conversion route, there are a handul of nice options:

When using the emaxx tranny, you essentially just have to dremel the chassis a little to mount it, then come up with some battery trays depending on what you plan to run exactly. When using the stock transmission & rcm mount, you have two options:

� Lock the tranny into 2nd gear, and use motor braking via the esc
� Keep the 2spd, and use mechanical brakes

If you choose option 2, which also applies to the TeknoRC option, you are also able to use a forwards only esc, such as a heli or boat model; you may also need to use the �PiStix� item, which converts the radio signal into something the esc can understand, as aero escs use different signal patterns to car/truck escs. In addition, when using either of these options, you are fairly limited to your battery mounting options, with only one side of the chassis free.

2. Transmission

E-Revo & BL E-Revo
The stock transmission has been greatly strengthened with the use of steel idler gears that are wider and stronger. The transmission comes locked into 2nd gear as standard, handy for those who use BL power in their trucks. An optional 2speed kit is available that reinstates 1st gear, with a choice of close and wide ratio gear sets; top speed remains unaffected.
The new transmission features a Revo spec slipper assembly that can be used with spurs ranging from 54tooth (jato 3.3) upto 68tooth that comes as standard in 32p (MOD 0.8). MOD1 spurs from the Revo can also be used provided a MOD1 pinion is also used- these are available from RC-Monster and RobinsonRacing as well as a few other places. The new transmission has been designed to allow the use of twin motors, as in the stock setup, or a single motor that can be mounted centrally- either with the stock motor mount or an option single motor mount and gear cover. The BL Erevo has the single motor mount & cover as standard & comes supplied with the CastleCreations NeuCastle 2200kv motor + 18/65 gearing. It is also worth noting that finned can motors (such as Neus) or larger diameter motors (Neus again, and KB45s) may be limited in their gearing range as the larger diameter cans may hit the transmission case before the pinion and spurs mesh correctly. With the lowered central motor mount there is the option to mount a large motor with a 30mm mounting pattern, such as the KB45 or larger brushed motors such as those sold by kershawdesigns and banebots etc.

Conversion
The stock emaxx 3906 transmission is a 2 speed unit that is shifted via a servo; when used in a revo conversion, keeping the 2sp option is difficult, but not impossible. There are a few weak points to the design however, mainly concerning the slipper mechanism and idler gears. The stock slipper uses small pegs inserted into the spur gear and a spring loaded pressure plate to control torque delivery; the problem is too much slippage causes the slipper pegs to melt, and the spur will just spin and spin, without sending any power to the wheels, or the slipper plate will overheat causing the spur gear to melt in the center. Simple solution is to tighten the slipper fully, though this does add more stress to the driveline resulting in broken driveshafts and diffs. Best solution is to skip the out-dated 3906 transmission & opt for the much better 3905 spec transmission. This is slightly narrower than the 3906 version so will require a small mounting adapter along one side in order to fix it to the chassis, relatively simple with some basic hand tools & some 3mm thick aluminium. GorillaMaxx also sell a single speed conversion kit that removes 1st gear and half the transmission case, an ideal way to save weight and eliminate the shifting mechanism, though that still leaves you with the weak plastic idler gear and slipper mechanism- the best option to fix this is to modify the transmission to use the 3905 spec slipper assembly (CLICK HERE). Then, add an Unlimited Engineering ( UE ) or Rc-Monster steel idler. The Erevo spec tranny has all the same features as the Emaxx 3905 tranny. The stock nitro revo trannies are essentially fine, though there are various retailers offering upgraded gear sets and such. Locking the nitro tranny into 2nd gear is a painless enough job, and requires minimal work- see the links thread for a 'How-to' guide.

3. Driveshafts

E-revo, BL E-Revo & Conversions
Very few people have reported issues with the new style of drive shafts, though they are still possible to twist and snap, especially in cold weather. Traxxas CVDs are a nice upgrade, though they themselves are not 100% bullet proof as the dogbone pin can snap under BL power. Being less aggressive on the brakes and throttle can reduce the amount of stress on these parts.

4. Bulkheads

In all my years of studying revos and revo conversions, I have only ever seen one or two broken bulkheads, and the new E-revo uses an improved design for even greater strength, so no worries there. The usual source of problems comes from the possibility if broken rocker posts, that rarely sometimes snap of, leaving a stump poking out of the bulk- cheap to replace.

5. Shocktowers

E-revo, BL E-Revo & Conversions
Since they only have to support the shell, they are much more durable and I�ve never seen a broken one, so again, no worries there. I would be inclined to say stick with stock or plastic models though, as aluminium versions will only transfer stress to more critical areas in a crash or flip.

6. Differentials

E-revo, BL E-Revo and Conversions
Traxxas improved the design somewhat with a Revo type sealed diff and additional spider gear support, but the cup and case are still plastic. Unfortunately there are no aftermarket cases that will fit at present, though FLM sell a metal cup that will remove the flex of the stock setup. The only real option for bullet proof E-revo or revo diffs at present are the RC-Monster V2 hybrid diffs, that use 1/8scale buggy or truggy diffs, and fit in the stock bulkheads. You will also require dogbone cups that fit an 8mm output shaft, or the RCM adaptor that allows you to use the stock center shafts instead of dogbones.
Another more complicated upgrade option is to modify ( dremel... ) the inside of the bulkheads to accept LOSI LST-2 or Muggy diffs- see the 'Useful Links' page for a link to some guides & examples.

7. Knuckles & suspension

E-revo, BL E-Revo & Conversions
Again, the stock design is pretty solid, though the pillow balls are known to pop out sometimes due to the soft plastic used in the knuckles- RPM knuckles are even softer it is worth noting, but they tend not to break so easily. Aluminium knuckles are a great upgrade, though this does remove a weak link from the truck, and will shift potential damage to other areas in a crash or cartwheel etc. RPMs rear suspension upgrade is good for people who don�t like messing with toe angles, as it locks the wheels to 1.5 degrees of toe-in, good for racing and all-round use generally.

The stock suspension arms are pretty sound, but RPM offer some nice upgrade items that are almost unbreakable. Ofcourse, there are also numerous aluminium options too along with Ti arms, it is a matter of personal taste as to which route you take, though RPM arms are very robust and ideal for people who are hard on their trucks. Set-back arms are a nice option also to increase the wheelbase, and reduce wheelies.
The stock E-revo comes with fairly soft suspension setup as standard; this can be improved by fitting stiffer springs- refer to the manual for specific details (yeah I know, but there you go :p) on setup guides. For off-roading action, use of the long travel rockers is a good idea, as it allows for more suspension travel, with greater ground clearance and smoother handling over the rough stuff.

8. Skid plates

E-revo, BL E-Revo & Conversions
The stock items are fairly sound, though there are also numerous aftermarket options for the nitro models, such as one piece �armour� that covers the whole underside. Ti is always my favourite material as it is strong, light, and flexible.

9. Steering mechanism

E-Revo, BL E-Revo & Conversions
The stock setup is fine essentially, the only real weak point being the weak gears in the servos, though this hasn�t been so much of an issue as compared to the 3905 servos that stripped due to a poor initial servo saver design, thankfully fixed now by traxxas. When looking at a new servo, try to find something with over 200oz/in of torque, be it digital or analogue. Metal gears will be more robust than nylon or Karbonite. There are numerous metal aftermarket models available, some of which aloe the use of dual, or a single high-torque servo.
The BL E-Revo comes with optional steering stops that allow greater side to side movement for the steering; why Traxxas doesnt just include this on the truck as-standard to begin with makes little sense, but never mind..

10. Wheels and tires

E-Revo, BL E-Revo & Conversions
The standard brushed setup uses a 14mm hex to mount the wheels onto the stubaxles. Aftermarket hexes and adaptors allow you to use anything between 12 and 23mm- 17mm and 23mm being the most common generally. The advantage here is that a larger hex diameter is less prone to stripping out inside the wheel- you will obviously require a wheel (or rim) that uses the same size hex as the one on the truck. For general use the stock chevron design tire is fine, though there are much better choices for onroad and offroad use, depending on the specific surface (hard dirt, loose soil, grass, tarmac etc). Proline, GRP, Maximizer are but a few of the aftermarket companies offering wheels, tires and hexes. Under BL power and high rpms many tires tend to �balloon� in diameter, which can cuase them to tear or fly apart, or at the very least cause loss of control. One solution is to use tire foams, and another is to belt the inside of the tire with thick duct tape (turn the tire inside out and run a couple of strips of tape around the tire, being careful not to overlap the edges which would cause imbalance). Traxxas themselves have introduced a 17mm hex and rims with the Revo Platinum, and this offers users yet another upgrade route.
The BL E-Revo comes with 17mm traxxas hex adapters and matching rims- these are the same 'toothed' hexes as used on the BL E-maxx, taping the insides of the tires to reduce ballooning and high-speed blowouts is advised.

11. EVX, EVX2 & MMM Speed controller

E-revo
Again, despite traxxas�s boasting the blue EVX2 is water proof, it is ill advised to test this too far, as traxxas will charge for repair of any moisture damaged escs. It suffers in general to a lack of voltage handling ability as freshly charged 6 and 7 cell packs can sometimes trip some form of over-voltage protection, though this isn�t the case with all EVX2s which is somewhat baffling. It is also recommended that you do not use the on/off switch at the end of a run, but to instead simply unplug the batteries from the esc. Any warranty issues should be dealt with by contacting the Traxxas customer service department.

BL E-Revo
The BL E-Revo comes complete with the brilliant CastleCreations MambaMonster ( MMM ) brushless esc and motor system. The esc is rated for 6s lipo or 18 nimh cells, the motor is the NeuCastle 1515 1y 2200kv model, based on the popular Neu of the same specification. The BL E-Revo comes with factory gearing of 18/65 which is good for about 35mph with 14 nimh cells or 4s lipo. With the optional 18/54 gearing 40mph is easily possible, but only with good quality lipos; nimhs will not provide sufficient current ( amps ) for the esc & motor, and esc damage may occur. This is due to something known as 'ripple current', whereby the voltage from the batteries fluctuates rapidly and drops as the current output increases beyond what it can provide due to high Internal Resistance ( IR ). Lithium batteries have much lower IR levels and so do not overheat or droop their voltage output under such strain ( provided they are of a suitable specification ). With 6s lipo & the correct gearing ( 24/54 ) it is possible to achieve a maximum speed of around 65-70mph, though aerodynamics & tyre choice become critical issues at this speed; strictly for on-road use only. It is very important when choosing lipos to select items with a good specification & that will provide sufficient current for the MMM to operate properly. The esc requires lipos capable of producing at least 120amps continuously, though the more overhead you have, the longer the batteries will live for and the longer your runtime will be ( as an indirect effect the voltage will not drop below the Low Voltage Cutoff ( LVC ) until nearer to the end of the discharge ). To determine how much current your lipos will produce, simply multiply the C rating by the mah capacity & devide by 1000, for example: 20c x 5000mah / 1000 = 100amps, or 40c x 5000mah / 1000 = 200amps. For best results the lipos should be of identical specifications & age/ discharges, though it is perfectly fine to use two different voltage/ cell counts in series, such as a 3s & 2s lipo to achieve 5s lipo ( I like 5s lipo as it is the perfect mix of power and speed when geared for about 40mph ). The MMM esc has numerous programming options & settings that are accessible via the 'CastleLink' programmer. This unit plugs into the rx lead from the esc & connects to a PC to allow changes to be made to various settings such as punch & torque control, LVC settings, timing and start power, brake strength and also throttle curves etc etc; the 3908 does not come with this unit so it is an optional extra costing approximately $25; well worth purchasing. Please see new Castlelink page for indepth info on how to use this device and the best settings.

12. Nimh batteries

E-Revo & Conversions
The best way to get good performance from any E-revo is to use good batteries, as these ultimately determine your runtime and the amount of �punch� & top speed your truck will have. When choosing packs, consider the amount of capacity they have in mah (Mila-amp Hours), the cell count/voltage, and the discharge rating if available. Higher mah will mean longer runtime, higher voltage will mean greater top speed, and a higher discharge rate will mean more punch along with more consistent top speed and acceleration. There are a great many brands out there, so choosing the best for your needs is somewhat tricky sometimes. In general, try to avoid cheap 6 cell shotgun or stick packs form Ebay, as these are assembled using thin tin strips to join the cells. Good quality brands are IB, EP and ENER-G, though there are many others. It is important not to overheat any battery, or to miss-treat it, as this can lead to ruptured cells or blown end-caps. Be sure to observe the correct polarity when building packs from scratch or adding cells to packs. 7cell side by side packs are best to achieve a good fit in the battery compartments, and for maximum speed.

BL E-Revo
CastleCreations themselves do not recommend using nimh batteries with the MMM esc as they cannot provide adequate current to meet the demands of the motor, yet at present Traxxas supply the BL E-Revo with a pair of 3300mah packs; go figure. These or better quality nimh packs will be sufficient for about 30mph on the stock 18/65 gearing- do not attempt to excede this figure as esc damage may occur; you have been warned.

13. Titan & NeuCastle motors

E-Revo
Older titans seemed to be more resilient to 16.8v use compared to newer titans, though this could just be a matter of perception; more internet users = more reports of failures. At any rate, it is important to use 19-23 turn motors with the E-revo and EVX & EVX2 escs on 14.4 to 16.8v. Using 12 turn motors on Emaxx voltages will result in burnt out motors and escs. There are various upgrades to look at when choosing new brushed motors for the Emaxx, from the $3.50 drill motors all the way up to expensive 19turn racing motors that need rebuilding after every run- the cheaper options tend to be more hardy and last a lot longer, and still give impressive results. KershawDesigns offers more options in the shape of larger motors, such as the 650, 700 HO and 970 sized motors, some requiring a custom motor mount and pinions in order to fit on the E-revo transmission. Performance gains vary, but weight gain definitely increases with the larger motors, and run time drops.

BL E-Revo
The BL E-Revo uses the brilliant NeuCastle 1515 1y 2200kv motor, based on the original Neu design of the same specification. The motors are made under license to the same spec as the original Neus, but there are various subtle differences that make them much better suited to use in ground based vehicles. The endbells are screwed in place & sealed to prevent the ingress of dirt and dust & to increase durability. The wires are no longer extensions of the windings from inside the motor, instead they are very flexible 10guage wires that can be shortened as required. The motor shaft comes with a large flat spot as standard to make attaching a pinion much easier ( though a drop of blue threadloc is always recommended ), and there is a shield inside the front endbell to prevent mounting screws that are too long from damaging the coating on the windings; screws no longer than 8-10mm are suggested, trim as required. To protect the motor wires, it is best to install the motor with the wires facing either downwards or off to one side, but not directly upwards- in the event you roll or flip the truck without it's shell on you would otherwise damage or strip the insulation from the soft wires. If you are looking for more runtime and cooler temps, it is a good idea to look at the NeuCastle 1518 1y 1800kv motor. This is slightly longer and a lower kv rating compared to the stock 1515 motor, meaning that when used with the same voltage and gearing it will result in a lower wheel speed and lower temperatures due to less current draw. When paired with 6s lipo and suitable gearing the 1518 motor makes for a very potent setup with outstanding torque, lower running temps & longer runtimes- a great motor for speeds in the 40-50mph region, and also for heavier trucks with a lot of after-market aluminium parts and/ or large & heavy tires. You will need to dremel a small amount of material from the rear shock-tower mount to allow room for the longer motor, but it is very minimal & wont harm the truck's durability.

14. V-twin concept

It�s actually remarkably simple really. All that is required is a pair of Traxxas VXL systems, a servo Y-harness, and 2 ounces of common sense & initiative. Bolt both motors to the transmission, and connect the escs to the receiver (rx) throttle channel via the servo Y-harness. Then, programme the two escs at the same time to your radio, and that�s it. The only difference here compared to the E-maxx V-twin is that one motor is reversed; simply switch any two of the wires from the esc to the motor around (outer two ideally, leaving the centre wire alone). You will need to use the same size pinion on both motors, and ensure the mesh is set correctly- use a piece of paper between the spur and pinion as you mesh them together, to prevent them binding too tightly. To get the best performance from a V-twin it is vital to use good batteries, this generally means a pair of 3cell (3s) lipos. To ensure even discharging on each system, it is a nice idea to use a parallel harness. It essentially is a pair of leads with male plugs at one end, and female at the other, with a wire linking the negative wire on each lead, and the positive leads in the same way. This ensures that the batteries discharge at the same rate, preventing the LVC from kicking in on either system before the other- see below:

CLICK FOR FULL SIZE

CLICK FOR FULLSIZE

Topspeed is around 55-60mph with good lipos and careful gearing, though it is important to keep a close eye on motor, esc and battery temps.

15. BL system upgrades

E-Revo & Conversions
The old Novak HvMaxx/HvPro systems are a nice intermediate upgrade, working well on both 12-14cells and a pair of 2s lipos with an LVC device. The Hv6.5 motor is seen as the best version, giving the best temps and good speeds of around 35-38mph. The Hv4.5 motor will give higher speeds of around 43mph, but at the expense of higher motor and esc temps- be careful not to thermal the esc as it can become permanently damaged. Next step up would be a HobbyWing EXRun 80amp esc & 4168 2200kv motor, this system giving more torque & speed potential whilst still having a good price-point. There is also the HobbyWing XERun & EXRUN 150amp escs which usually come bundled with the 4274 2000kv motor; this system gives near-MambaMonster levels of performance with a maximum voltage of 6s lipo possible. It is also entirely possible to build a custom system basically, using a MambaMaxpro (MMPro) esc and a Feigao-type L or XL can motor, along with an external BEC/UBEC (Battery Eliminator Circuitry, powers the Rx and servos). The MMPro will happily run on 6s lipo, though in a heavy MT it is best to restrict the voltage to 4s lipo. The best motors to use would be either the 9L or 10L to get good speed, and the 8XL or 9XL for good speed and more torque. Using a motor with fewer turns/higher KV rating (rpms per volt) will result in higher temps and potential damage to the motor, esc and batteries. 100-180f is generally seen as the safe zone for motors and escs, with 135f or below being optimum.

There is an old rule of thumb regarding temps which many will find handy no-doubt:

As a general rule this is fine, though it is possible to get different results depending on the esc and motors used- some motors run hot regardless, and some escs are less capable than others to deal with high current draw. It is important to choose a suitable sized and KV motor for MT use- 36-39mm diameter, and 60-80mm in length is a good guide. The KV rating will depend on your voltage, but as a general rule again:

If you are planning on using more than 6s lipo, chances are you don�t need to be reading this FAQ�
Beyond that there are higher end escs such as the Quark 125b, MambaMonster (MMM), Kontronic Jazz, Schulze, Hacker mastercar, MGM, and so on. Ideally, you want an esc that can handle upto 6s lipo or more, and is rated for atleast 120amps ideally.
High end motors tend to take the shape of Neu 1515 or larger models, Lehner 1940 and larger models, Hacker C50maxx and Feigao XL models, and the Castle-Neu MMM motors, along with the Medusa 60-80mm models, and Megamotor 22/45 series. There are numerous other good quality motors these days that are suspiciously similar to the Neu & NeuCastle motors, such as the offerings from Hobbywing & Leopard motors- good quality motors, but using lesser magnets than the higher quality motors; exploded rotors & higher running temps are likely if you arent careful with the gearing & kv/ size choice for your application. The Plettenberg BigMaxximum used to be concidered the very best of the best, but these days this 4 pole German monster is very overpriced & has a kv rating much too high for use with anything other than 4s lipo- does not make for a very efficient setup at all.
The stock radio system does a reasonable job with the stock setup, but BL systems tend to create a lot of RF (radio frequency) electrical noise, and Am radios do not react well, with glitching and loss of range & control a common problem. Best solution is to upgrade to an FM radio at the least, or a digital radio is even better, budget permitting.

16. Lipo and A123 batteries

Lipo = lithium polymer. They have a nominal (resting) voltage of 3.7v per cell, and a fully charged voltage of 4.2, but unlike Nimh cells, you can actually select the fully charged voltage provided you have a suitable charger; this is handy for escs that don�t like peaked voltages sometimes (MambaMaxx and Quark 125b for example).
When charging, and discharging (running them in a vehicle), the individual cells in a pack can become out of balance. This means in a 2s pack for example (s = cells in series), one cell can read slightly higher or lower than the other, and over time this difference can become large enough to mean that one cell becomes over-discharged during use; it is important not to discharge them below 2.8v per cell, but 3.2v is a safer level.
This is where the LVC device comes into play, as it cuts the throttle, or reduces power when the cells have drained down to whatever the cut-off voltage is set to. Some LVCs have adjustable voltage settings, and cutoff type (reduced power, warning light, warning buzzer, total shut-off etc), depending on the application; you wouldn�t want to loose all power with a plane or heli for example...
When charging lipos, the balancer device is used to keep the voltage equal in all the cells of a pack- it can be a balancer that works along with the charger, separate from the charger, or you charge through the balancer. The end result is the same though- cells with a lower voltage get extra charge, whilst cells with a higher voltage get drained a little to keep them in line with the others. This is what the little white plug is for that you see sticking out of lipo cell packs, it is the balancing plug/tap.
The lipo�s battery chemistry requires a special technique, called the CC/CV method (constant current/constant voltage). Basically, the charger will pump in a high current (amps) until the cells reach ~4.2v, then they will drop the current but maintain the voltage level until the cells reach and stay at 4.2v, at which point they are fully charged. Over charging is very dangerous, as lipo cells contain alot of energy, and overheating them can be a serious firehazard, hence the items called 'lipo sacks' that should be used to contain any possible damage caused by a burning lipo. The chances of that are rare these days though, and only idiots on youtube tend to burn up lipos for fun.
Lastly, the C rating thing. This refers to a lipos maximum discharge rating, and is a multiple of its capacity. For example:

A 20c 5000mah pack can put out 100amps.
A 10c 8000mah pack can put out 80amps.

Choosing the right capacity and C rating is important for any given application, and should be based on how many amps your chosen motor can draw at maximum load. For example, the feigao XL motors tend to draw about 80-100amps at peak current levels, so you should select a battery that can supply at least 80amps continuous. Generally though, the motors pull a continuous current of about 10-20amps or thereabouts, so the battery wont see very high drain rates for the whole run, otherwise it would soon over-heat and 'puff'. Once a battery puffs, its pretty much toast, though individual cells that are still okay can be salvaged- a 5s lipo can be reconfigured into a smaller pack of 4s etc. It is worth noting though that high mah capacity lipos with a low C rating at not as capable at discharging large amounts of current compared to a smaller capacity lipo with a higher C rating, this is because the voltage level droops heavily under a high load, due to the lipos low tolerance to high current draw; a higher C rating is always preferable regardless of the packs actual capacity. With the E-Revo, you are limited to a 8000mah 2s lipo, or 5000mah 3s lipo in each battery compartment, though specific sizes of packs vary, see below:

EREVO BATTERY COMPARTMENT MEASUREMENTS ( CLICK FOR FULL SIZE )

A123 cells have a very high energy density like Lipo cells, but are inherently safer and 99% indestructible, so do not have to be used with an LVC (low voltage cut-off) device, and can be charged much quicker given the correct equipment is used. Their resting voltage is 3.4v, which makes a 5s configuration ideal for most applications that would normally use 12~14 Nimh cells, or 4s lipo. The downside is that they are limited to 2300mah or so of capacity, so ideally should be ran in a parallel formation such as 5s2p (2 five cell packs connected in parallel to double the Mah capacity but maintain the same voltage as 5 cells). They can be purchased easily by buying a Dewalt 36v battery pack on Ebay, and then following one of the online tutorials which demonstrate how to turn them into RC packs. Mounting them in the E-revo requires some fairly drastic dremel work however.

A new generation of prismatic LifePo4 (A123 chemistry) cells are currently hitting the market. These offer all the benefits of lipo cells in terms of power density and weight/size, but also have the benefit of safer chemistry, faster charge rates, no LVC required and capable of very hig current output without puffing or swelling. Currently there are only a handful of vendors for these new batteries; MaxAmps, HobbyKing/HobbyCity, and Protek being then main notable sources, though given the extortionate mark-up that MaxAmps charges (to pay for all its advertising and sponsorships no-doubt) I would stay clear of them until their prices drop inline with the competition... if they drop that is. There are other similar cells on the market, but most are low discharge, high capacity packs, unsuitable for R/C use. Time will tell on this type of battery, but if other companies start making and selling them for the R/C market, things will be alot brighter.

17. Chargers and balancers

Its important to choose a good battery charger that is suitable for charging your type of batteries. Do not go cheap, as you will inevitably want to upgrade to a better & faster charger in a short space of time. Ideally, you will buy a charger that can handle both lithium based and Nimh cells, that way you are ready for a lipo or A123 upgrade in the future without having to buy a new charger. A good charger will allow you to charge upto 14 cells or 6s lipo, and charge at a 1c rate (1c = batteries mah capacity, 1amp = 1000mah). This is important as some chargers have a lower wattage rating than others, and therefore must lower their charge current (in amps) when charging a high voltage battery pack, in order to stay within that wattage rating, EG:
A 50watt charger will drop to around 3.5amps when charging a 4s (14.8v) lipo 4000mah pack.
To workout how powerful your charger needs to be to charge a pack at 1c, simply multiply the fully charged voltage of your biggest pack, say 6s lipo (25.2v) by its capacity in amps, say 5amps (5000mah), which gives you 126watts. You will therefore need a charger that is rated for about 120watts or more, though generally speaking 50-100watts is more the norm that you will find in terms of mains powered chargers.
If your charger is a DC type, it will need a power supply that is capable of outputting sufficient amps to power the charger, same rules apply basically when looking at power supply specs. An old PC is a good source for a good strong power supply, though it will require some modifications to serve its new function. Finally are balancers, which were covered before in section 16, but there is a little more to add. When choosing lipo or A123 battery packs with balance taps, it is important to consider the type of tap they come fitted with, as there are a dozen or so different brands/styles, and you may need to find a suitable adaptor so that your lipo battery can be balanced by your balancer. My advice is to choose the most common brand of tap when you are given the option (some stores allow you to choose the brand of tap you want fitted to your packs), and always choose that type whenever you buy new batteries (see below for a compatibility chart). It is no great hardship to rewire a lipo pack, or to find a suitable adaptor, it just requires a lot of searching around and head scratching usually, but that is the nature of the hobby really; welcome to the wonderful world of RC.